1. Field of the Invention
This invention relates generally to digital signal processing and digital communications, and more specifically to modulation using digital-to-analog converters and frequency multipliers.
2. Description of the Related Art
It has become increasingly important to reduce the size, weight, and power consumption of electronic devices, especially personal communication devices such as cellular telephones. Consumers want devices that are easily carried around and have long battery life. Electronics manufacturers have therefore sought to reduce the number of components in electronic devices by using components that perform multiple finctions.
Modulation is a function that is often performed in consumer devices. This is especially true for consumer devices that transmit signals over long distances, devices that transmit or store data represented as digital signals, and all types of telecommunication devices. Data signals are rarely transmitted over a distance in their raw form. The signals are normally modulated to match their frequency characteristics to those of the transmission medium in order to minimize signal distortion, to use the available bandwidth efficiently, and to ensure that the signals have some desirable properties. The process of modulation often involves modifying a high frequency signal, known as the carrier signal, in sympathy with a data signal, called the modulating (or baseband) signal. For convenience, persons skilled in the art typically categorize modulation into one of two categories.
The first modulation category is called data modulation. Data modulation refers to the process used when encoding a stream of data (often referred to as a baseband signal) into a carrier signal. The way in which the carrier signal is encoded is called the modulation scheme. Commonly used modulation schemes for transmitting analog data over a bandpass channel (e.g., a microwave, cellular telephone, or other radio frequency link) include, for example, Amplitude Modulation (AM), Frequency Modulation (FM), Single Sidebanid Modulation (SSB), etc. Commonly used modulation schemes for transmitting digital data over a bandpass channel include, for example, Amplitude Shift Keying (ASK), Phase Shift Keying (PSK), and Frequency Shift Keying (FSK), etc. A carrier signal modified by the modulation process is known as a modulated carrier.
The second modulation category is called up-conversion and is used to refer to the process of shifting a signal from one frequency range to another (usually higher) frequency range. For example, in commercial broadcast applications, such as standard AM, FM, or television broadcasts, a radio program (the baseband signal) is used to modulate a radio frequency carrier signal. The modulated radio frequency carrier signal is amplified and provided to a transmitting antenna that radiates the amplified modulated radio frequency carrier signal as electromagnetic waves (radio waves). The electromagnetic waves may be received by a transistor radio and demodulated to recover the original radio program, which is then played on a loudspeaker. The baseband signal (the radio program) is an audio frequency signal, usually having frequencies in the range of 20-20,000 Hz. The radio frequency carrier is a radio frequency signal, often having a frequency of 50 MHz (megahertz) or higher.
The perceived differences between data modulation and up-conversion are largely illusory, and the distinction between data modulation and up-conversion is generally used merely for convenience. Thus, one skilled in the art will recognize that techniques used for data modulation can also be used for up-conversion, and vice versa. The term up-conversion is used herein as a convenient way to describe the process of using a modulator to shift the frequency of a signal from a lower frequency to a higher frequency.
When the data signal and the carrier signal are both analog signals, then a modulator may be constructed using an analog multiplier (also known as a mixer) having two inputs and one output. The data signal is provided to the first input and the carrier signal is provided to the second input. Given a passband signal G(.omega.), centered at .omega..sub.0, and a carrier signal cos(.omega..sub.c t), then the output H(.omega.) of the multiplier is a signal having two sidebands. The spectrum of the signal H(.omega.) is the combination of two terms known as upper and lower sidebands. The frequency components of the lower sideband correspond to the sum .omega..sub.c -.omega..sub.0 and the frequency components of the upper sideband correspond to the difference .omega..sub.c +.omega..sub.0. The multiplier is said to create sum and difference frequencies. The signal H(.omega.) may be filtered to remove one of the sidebands.
Alternatively, the data signal g(t) may be represented by a digital signal g(k). The signal g(k) is a sequence of digital values, such as the output of an analog-to-digital converter, where k is an integer index which identifies the k.sup.th value in the sequence. When the data signal and the carrier signal are both digital signals, then a digital multiplier, having an output h(k) may be used as a modulator. The output of the digital multiplier is a digital signal which may be converted to an analog signal by providing the signal h(k) to an input of a digital-to-analog converter.
Manufacturers have sought to produce modulators that are more power efficient and lightweight. For example, a typical modulator used in a cellular telephone may comprise a digital-to-analog converter to provide the data signal, a first filter to remove unwanted sidebands produced by the digital-to-analog converter, a local oscillator to provide the carrier signal, a mixer to create the sum and difference products (sidebands) and a second filter to remove unwanted sidebands produced by the mixer.
The first filter is used to remove harmonics produced by the digital-to-analog converter because the analog signal produced at the output of the digital-to-analog converter comprises an infinite number of harmonics. Each harmonic corresponds to a harmonic frequency of the frequency at which digital values are provided to the analog-to-digital converter. The harmonics occur at frequencies given by n.omega..sub.s, where n=.+-.1 . . . +.infin. and .omega..sub.s is the sample frequency (i.e., the rate at which the digital-to-analog converter produces a new output value). Thus, a digital-to-analog converter, in addition to converting a digital signal to an analog signal, can act as an up-converter and convert a signal from baseband to a large number of harmonics.
Prior art attempts to minimize the number of components in the modulator generally result in increased distortion of the modulated signal. This increase in distortion adversely affects the modulation accuracy of the system, it adversely affects the power efficiency of the system, and it adversely affects the spectrum of the modulated signal.
The modulation accuracy of a system is usually specified in terms of the total allowable modulation error, expressed as an error budget. The error budget generally arises from industry specifications and government regulations. The error budget is "spent" by tallying the error (or distortion) introduced by each stage in the system. Errors produced by one stage of a system usually propagate through the system and may even increase the errors produced by later stages in the system. Thus, if one stage introduces a relatively large error, that large error must be offset by a relatively smaller error in other stages.
Distortions produced by the modulator often reduce power efficiency of the system because the distortions cause an increase in the amplitude of unwanted harmonics of the modulated signal. The power amplifiers, especially the transmitter power amplifiers, amplify these unwanted harmonics. In order to meet the total error budget of the system, the power amplifier is typically operated in a more linear region. Unfortunately, many RF power amplifiers are less efficient when operating in the linear region, and thus, linear operation may increase the drain on the batteries. Distortions, such as, for example, sinc distortion, in the modulator may also reduce the amplitude of desired harmonics of the modulated signal, resulting in a need for additional amplification, which also increases the power consumption of the power amplifiers, thereby diminishing battery life.
Finally, distortions produced by the modulator adversely affect the spectrum of the modulated signal in ways that often require additional filtering to correct (e.g., filters with steeper slopes and more filter stages). Additional filtering is often needed because the distortion may cause a relative increase in the amplitude of unwanted harmonics and a relative decrease in the amplitude of desired harmonics. The additional filtering is used to reduce the amplitude of unwanted harmonics and increase the amplitude of desired harmonics.
One type of distortion that is often found in a modulated signal is distortion due to a phenomenon known as the sinc effect (discussed below in connection with FIG. 3). The sinc effect causes a distortion of the modulated signal that adversely affects the error budget. When modulators of the type described above are incorporated into electronic devices, their relatively large inaccuracy uses up a large portion of the error budget, and attenuates the desired harmonics relative to the undesired harmonics.